Gold and ruthenium plating baths



United States Patent 3,530,049 GQLID AND RUTHENIUM PLATING BATllS Julius Scherzer, Baltimore, Md., and Alfred M. Wersberg, Providence, R.I., assignors to Technic, Inc., Cranston, R.I., a corporation of Rhode Island,

No Drawing. Original application Mar. 15, 1965, Ser. No. 439,988. Divided and this application Oct. 2, 1968, Ser. No. 764,593

Int. Cl. C23b /42, 5/24 U.S. Cl. 204-43 3 Claims ABSTRACT OF THE DISCLOSURE An aqueous electroplating bath for depositing ruthenium comprising ruthenium as a chelate of ethylene diamine tetraacetic acid, nitrilo triacetic acid or cyclo hexane diamine tetraacetic acid. The above chelates also are used as grain refining agents in acidic gold plating baths.

RELATED APPLICATION This application is a division of our corresponding application filed Mar. 15, 1965, Ser. No. 439,988, now abandoned.

This invention is concerned with the reparation of electroplating baths employing ruthenium as the metal to be plated and employing it in the form of some new ruthenium chelates. More particularly, the invention is concerned with the preparation of and use of electroplating compositions employing the ruthenium chelates of synthetic amino acid chelating agents, which include nitrilo triacetic acid (NTA), ethylene diamine tetraacetic acid (EDTA), and cyclo hexane diamine tetraacetic acid (CDTA), and also the mono and di-hydroxy-alkyl variants of these compounds, i.e., ,B-hydroxy ethyl ethylene diamine triacetic acid; and N,Ndi-,8-hydroxytin ethylene diamine diacetic acid.

Ruthenium is a metal, actually rare, but having very valuable properties for industrial purposes. as well as ornamentation, when it is applied to surfaces as an electro deposit. It has limited use in manufacture of jewelry, but has substantial use because of its physical properties in the aerospace and electronics industry. This is because it is like gold in its resistance to tarnish, corrosion, marring and the like, and also has excellent high temperature properties.

The usefulness of the several ruthenium chelates as agents for electroplating of ruthenium and, actually, electroless plating, as well as refining agents in the electroplating of other noble metals is an unusual effect not determinable without experimental observation. We have found that electroplating baths, or certain baths containing the ruthenium chelates of EDTA, CDTA and NTA and their variants can be used for the electro deposition of gold, the electroless deposition of gold and as gold deposit refining agents in gold plating baths.

The term refining agent refers to the effectiveness of the ruthenium compound in interfering with the crystalline structure of the gold being deposited, thereby to give a fine grained surface deposit on the finished article.

The compounds are useful as electroplating agents over a broad range of pH for the deposition of ruthenium and, as refining agents, improve the plating properties of other electroplating baths. It is also possible, using the ruthenium chelates to introduce ruthenium as an alloying metal into the electroplating baths.

Accordingly, it is a basic object of this invention to provide a group of novel ruthenium compounds, finding their immediate usefulness in electroplating, for the electro deposition of ruthenium, for the alloying of ruthenium with other plates such as gold and other precious metals,

Patented Sept. 22, 1970 and for use as refining agents in gold plating baths.

Other objects and advantages of the invention will in part be obvious and in part appear hereinafter.

The invention, accordingly, is embodied in a group of chelate compounds, which are the reaction products, or the ruthenium chelates, formed with ethylene diamine tetraacetic acid, cyclohexane diamine tetraacetic acid, and nitrilo triacetic acid, both in the acid or alkali metal salt form, the compounds thereby being the reaction products of soluble ruthenium salts with these acids so that the resulting chelate compounds contain ruthenium and the radical of the chelating agent. In some cases, besides the radical of the chelating agent, the compounds contain acid radicals, hydroxyl ions and/ or water molecules. The general formula for the compounds may be stated as follows:

X, Y=anion M=Na, K, NH 1, m, n, p, x=0, 1, 2

In general, the compounds with acid character (12:0; 1:0) are slightly soluble in water, virtually insoluble in methanol, ethanol, acetone, and other common organic solvents. In view of the solubility of the alkali or ammonia salts of the compounds and the low level of ruthenium ion concentration obtained from the complex chelate form, the compounds lend themselves favorably to the preparation of electro deposition baths in which the ruthenium can be deposited under electric current or it can be deposited Without the electric current in the electroless versions, the problem being one of inducing a higher level of ionization for the electroless solution. Under the conditions when the ruthenium is only very slightly ionized, it is possible to use the ruthenium chelates as brightening agents in substantially conventional gold bath formulations and others.

Hence, the invention in the instant case consists of the preparation of the ruthenium chelates, the compositions prepared therefrom, and the technique of their use in the formulation of plating baths.

The invention will be better understood by reference to the following detailed discussion of the preparation of the compounds and their application to the preparation and use of electroplating baths.

In order to prepare the ruthenium chelates mentioned above, the ratio of reactants can be varied, but the preferable ratio is one mole chelating agent per mole of ruthenium salt. The reactions occur in aqueous solution and by working with acid solutions, the ruthenium chelates separate consecutively, in time, as intensively colored precipitates. In concentrated solutions, the precipitation of the compounds starts a few minutes after the mixing of the reactants, but the complete precipitation of the different and resulting ruthenium chelates takes sometimes several days. In some cases, the precipitation can be accelerated by adding a water miscible organic solvent to the solution.

The reaction between ruthenium salts and the chelating agents mentioned above can be carried out in aqueous solution either at room temperature or at higher temperature (up to C.), but the resulting compounds are different, particularly the first solid products obtained.

By dissolving the precipitated ruthenium chelates in aqueous solutions of bases, adding a suitable conducting or buffering salt (for instance, disodium phosphate) and a suitable acid or base to adjust the pH, there results a solution which can be used as a plating bath for ruthenium. The concentration of the bath in ruthenium metal may be in the range of 0.150 grams ruthenium per liter. The bath may be operated at a current density of 10-600 amperes per square foot (a.s.f.) with optimum results at 100300 a.s.f. Agitation may be employed, though it is not necessary. The bath may be operated at 100-200 F. with optimum results at 150-180 F. The pH range for operating this bath would be 7.512 with optimum results at a pH of 910.

The following nonlimiting examples are illustrative of the present invention:

EXAMPLE I A solution of 10 grams ruthenium chloride, dissolved in 100 milliliters water, was added, under stirring, to a solution of 19.5 grams disodium salt of the ethylene diamine tetraacetic acid, dissolved in 150 milliliters water. A dark red-brown solution resulted. The fresh prepared solution should be acid (pH about 2.5). The solution was heated to 80 C., when it started to precipitate a black-brown substance. After 2 hours of heating, the solution was filtered. The black-brown precipitate was washed with hot water, then with methanol and dried in vacuum. The analysis of the dry substance gave the following results: N, 6.10%; Cl, 7.54; Ru, 22.01. Calculated for RuC1 H 4O N2CL2H2OZ N, C], Ru, 21.84%. The infrared spectra revealed the presence of stretching frequencies of the OH and carbonyl groups. The substance is slightly soluble in water, but can be easily dissolved in a solution of alkali or ammonia hydroxide, resulting in production of a water soluble alkali (or ammonia) salt. This salt can be precipitated from aqueous solution by addition of methanol or ethanol in excess. It can be purified by recrystallization from water and methanol. In properties, reaction with solution of bases and analysis it agrees with the following formula:

Ru (H EDTA) Cl 2H O EXAMPLE II To the filtrate, obtained after separation of the brown ruthenium chelate of Example I was added about 650 milliliters methanol. A greenish precipitate resulted. Keeping the solution over night (10 hours), it precipitated a further amount of substance which was filtered, washed with methanol and dried in a vacuum desiccator. The dry substance with greenish-yellow. The analysis gave the following results: N, 6.86%; H, 3.76%; Ru, 24.48%. Calculated for RuC H O N .H O: N, 6.86%; H, 3.67; Ru, 24.73%. The following formula is indicated:

[RuH O (H.EDTA) 1 The compound is easily soluble in solutions of alkali or ammonia hydroxides, forming the corresponding salts,

which can be precipitated from aqueous solution by addition of methanol or ethanol.

EXAMPLE III 2.9 grams nitrilo triacetic acid (NTA) were dissolved in a beaker in 60 milliliters of water, in the presence of 2.5 grams potassium hydroxide. In another beaker were dissolved 4 grams ruthenium chloride hydrate in 25 milliliters water. The ruthenium chloride solution was then introduced in the solution of the nitrilo triacetic acid (potassium salt) and the pH of the solution was adjusted to 5 with potassium hydroxide. Keeping the solution several hours at 90 C., this forms a brown precipitate of a ruthenium-NTA chelate. This chelate is soluble in an aqueous solution of potassium hydroxide.

In the filtrate, resulting after separation of the brown ruthenium chelate, there could be separated a green ruthenium-NTA chelate by addition of alcohol.

EXAMPLE IV An electrolytic plating bath for ruthenium is made by preparing a solution, containing the following components:

5 grams per liter of ruthenium metal as ruthenium- EDTA chelate, dissolved in a 20% solution of potassium hydroxide.

grams per liter of potassium phosphate, (dibasic) and monobasic, taken in equal parts.

Concentrated phosphoric acid, to drop the pH of the basic solution down to value 9-10.

Water, to make the balance to one liter solution.

The solution was heated to 160-180 F. and working at a current density of 3050 a.s.f., it resulted in a bright steady, chemical resistant ruthenium deposit. Slower but equally good chemically resistant plates are obtained at lower temperatures extending down to room temperatures.

EXAMPLE V Similar electroplating baths for ruthenium can be prepared by making a solution using the ruthenium NTA chelate, the other components consisting ofthe phosphate, water and alkali metal hydroxide to adjust pH, are substantially of the proportions given in Example IV.

EXAMPLE VI An electroplating bath for ruthenium is also made using the ruthenium chelate of cyclo hexane diamine tetraacetic acid with the phosphate and hydroxide used as buffers for pH adjustment.

EXAMPLE VII A gold plating bath, for example, a conventional bath of the following composition:

G./ 1. Potassium citrate 75 Citric acid 55 KH2PO4 Gold 7.5

Add H-Ruthenium EDTA (24% solution) 0.5

Operate Temperature 150 Current density ASF 120 pH 4-9 Current efiiciency 90% plus.

Alter pH with KOH solution To this conventional bath may be added amounts of ruthenium, extending from gram per liter to 5 grams per liter of ruthenium metal, as outlined in Examples V and VI and the solution used for plating gold. Ruthenium can be at very low concentrations and, if the pH is at an acid level, 3.55, there results a fine grained gold plate characterized by the fact that it is bright, unusually fine-grained gold with substantially no ruthenium present in the plate. As the pH is raised into higher levels and into the alkaline range of 7-11, the ruthenium plates out with the gold to give a ruthenium alloy, also however with a refined grain structure.

From the foregoing examples and description, it is apparent that each of the above mentioned chelating agents can be used to prepare several ruthenium chelates, which can be isolated from solution using either a difference in solubility or a difference in the rate of formation. Most of these compounds have acid properties and can be easily transformed to water soluble alkali or ammonia salts. The ruthenium chelates are stable in a broad range of pH, either in acid or in basic solution, even at high temperature. These properties enable the use of these chelates for different practical purposes. The use of some of them as plating agents has given good results. Their stability in solution at different pH values and at high temperature in the presence of additional substances used in electroplating baths, permits the deposit of adherent, bright and resistant ruthenium deposits on different substrates.

The usual substrates for plating are brass, and copperplated brass, or white metal, and copper-plated white metal, or various aluminum alloys and the like carrying copper plate when costume jewelry is being made. The substrate is any of these inexpensive metals which can be cast into good sharp form to take the patterns desired. It is then plated with nickel and copper, and finally With gold or ruthenium, depending upon the ultimate finish desired.

What is claimed is:

1. In an aqueous electroplating bath for depositing gold at a pH level of from 3.5 to 5.0, the improvement comprising including a ruthenium chelate of a compound selected from the group consisting of ethylene diamine tetraacetic acid, nitrilo triacetic acid and cyclo hexane diamine l0 tetraacetic acid in amount from 0.1 to 5.0 grams of ruthenium per liter of bath as a grain refining agent.

2. An aqueous electroplating bath consisting essentially of (a) a ruthenium chelate of a compound selected from the group consisting of ethylene diamine tetraacetic acid, nitrilo triacetic acid and cyclo hexane diamine tetraacetic acid, in an amount sufiicient to place 0.1 to 50 grams of ruthenium in solution per liter of bath, (b) a conducting concentration of a phosphate conducting agent, and (c) a concentration of acid or alkali sufficient to adjust the pH to a level between 7.5 and 12.0.

3. The aqueous electroplating bath of claim 2, consist- -6 ing essentially of (a) a ruthenium nitrilo triacetic acid chelate, (b) a phosphate conducting agent and (c) an alkali metal hydroxide.

References Cited UNITED STATES PATENTS 2,724,687 11/1955 Spreter et al 204-44 XR 3,290,234 12/1966 Parker et a1. 204-47 FOREIGN PATIENTS 1,331,063 5/1963 France.

OTHER REFERENCES Latimer et 211., Reference Book of Inorganic Chemistry, MacMillan Co., New York, N.Y., p. 412 (1940).

HOWARD S. WILLIAMS, Primary Examiner G. L. KAPLAN, Assistant Examiner US. Cl. X.R.. 

